Introduction
X-ray Imaging Dose to Patients in the Era of
Image-Guided Radiation Therapy
Image-guided radiation therapy (IGRT) has dramatically
improved the accuracy of radiotherapy
IGRT has emerged as the new paradigm in radiotherapy.
George Ding, Ron Price, Charles Coffey
Vanderbilt-Ingram Cancer Center
Vanderbilt University Medical Center, Nashville, TN
X-ray imaging, such as cone-beam CT (CBCT), for patient
setup can add radiation dose to patients.
Additional imaging dose may entail biological risk
How much are the imaging doses to patients?
CE: AAPM 2008, 7/31/2008, Houston, TX
Electronic portal imaging device (EPID)
Modalities used in imaging guidance
electronic portal imaging device (EPID)
kilovoltage digital radiography (kV DR)
megavoltage cone-beam CT (MV-CBCT)
kilovoltage cone-beam CT (kV-CBCT)
CT-on-rails
Flat panel
detector
MV AP setup field (Head and Neck)
MV AP setup field
MV AP setup field (double exposure)
MVCT on Linac unit
MVCT images
MVCT images
MVCT on Tomotherapy unit
Images of MVCT Tomotherapy unit
CT-on-rail system
Images of MVCT
Tomotherapy unit
kV CBCT system
kV CBCT system
kV Lat setup field
kV CBCT application for SBRT- Prostate
kV CBCT application for SBRT- Lung
kV CBCT application for SBRT- spine
Monitoring treatment progress and ART
Information from 2D images
2D projected verification images:
– Bony anatomy relative to treatment isocenter
– Seeds position relative to treatment field
– Two orthogonal project images for 3D positioning
– Treatment field shaped by MLC
Feasibility of kV CBCT for treatment planning
Information from images
3D images from cone-beam CT:
– Can be viewed in axial, coronal, and saggittal
reconstructions from the volumetric images
– Organ shape and position relative to isocenter
– Small lung tumor
– Can be used for monitor treatment progress and
adaptive radiotherapy (ART)
Radiation doses
Radiation doses cont…
Detailed list see AAPM TG-75
2D projected verification images:
2D projected verification images:
– kV beams from OBI device ( ~ mGy/field)
– MV beams EPID ( 1-2 MU ~ 1- 2 cGy/field)
• Setup fields: two orthogonal beams
• Setup fields: two orthogonal beams
• mAs can be automatically adjusted (patient size
dose )
• Treatment portal field
• Dose rapidly decreases as a function of depth in patient
– can be acquired during treatment (no additional dose)
– Before or after ( can be added to total dose)
• Double exposure (area larger than the treatment field)
Radiation doses cont…
CBCT volumetric images:
– scan area is larger than the treatment field (~cGy/scan)
– Standard acquisition mode
– Low-dose acquisition mode
– For the same scan (patient size
dose )
Dose dependency on depth
Dose dependency on medium for kV beam
Dose dependency on medium for MV beam
220
110
90
70
Bone slabs in water
160
Relative dose
Relative dose
180
Monte Carlo
Density corrected
80
125 kVp
200
6 MV
100
60
50
40
140
Monte Carlo
Density corrected
120
100
80
60
30
40
Bone slabs in water
20
20
10
0
0
0
2
4
6
8
10
12
14
16
18
20
0
2
4
6
8
10
12
14
16
18
20
depth in phantom /cm
depth in phantom /cm
Slab of water
Isodose distributions: single AP X-ray (CBCT beam) no bow-tie
20 cm thickness
A
X-rays (125 kVp)
125 kVp x-rays used for CBCT from a Varian Trilogy
B
Isodose distributions: single AP 6 MV beam
Dose profiles along the line A-B on chest CT: kV vs. MV
350
A
Single AP field
Sternum
(bone)
300
6 MV beam
125 kV beam
Relative dose
250
200
Vertebrae
(bone)
150
100
B
50
0
0
A
µen/ρ /(cm2/g)
1
0.1
0.01
0.01
0.1
4
6
8
10
12
14
Depth
/ cmin patient /cm
central-axis
depth
16
B
18
Dose distributions resulting from a kV CBCT
Head and Neck
Bone
Water
EYE
Lung
Brain, Grey/White matter
Tissue, soft
Blood
Air, Dry
10
2
1
Energy of photons /MeV
M U E n8
Data are from: J. H. Hubbell and S. M. Seltzer, "Tables of X-Ray Mass Attenuation Coefficients and
Mass Energy-Absorption Coefficients," National Institute of Standards and Technology, Gaithersburg,
MD NISTIR 5632, 1995.
Results of dose-volume-histogram analysis for H&N scan
100
Adult
Cord
90
Half-Fan mode
% volume
80
30
30
Line A
Line B
25
Line C
Line D
25
Eye
70
Cervical vertebral
60
Brain
50
40
Body
dose /cGy
15
20
20
15
10
10
10
0
5
5
0
5
10
-15
-10
-5
0
5
X direction /cm
10
15
-15
-10
-5
0
5
10
15
20
25
30
dose /cGy
0
0
15
X direction /cm
Dose distributions resulting from a kV CBCT
Chest
Results of dose-volume-histogram analysis for chest scan
100
Adult chest
90
Lt lung
80
% volume
dose /cGy
30
20
70
Bone
Rt lung
60
50
40
30
Cord
Body
20
10
0
0
5
10
15
dose /cGy
20
25
30
Dose distributions resulting from MV-CBCT: pelvic scan
Results of dose-volume-histogram analysis for a pelvic kV CBCT scan
Large Adult
100
Adult
90
% volume
80
Femoral head
70
60
Body
50
40
Rectum
30
20
Prostate
10
0
0
5
10
15
20
25
30
dose /cGy
Summary
Commonly used image-guided procedures
MV imaging:
– megavoltage electronic portal imaging (MV-EPI)
– megavoltage cone-beam CT (MV-CBCT)
• On Linac unit
Summary cont...
Doses from image-guided procedures
MV imaging:
– MV-EPI: ~ 1-2 cGy /acquisition
– megavoltage cone-beam CT (MV-CBCT)
• Linac unit: ~ 5 – 20 cGy /acquisition
• Tomotherapy unit: 8-12 cGy
• On Tomotherapy unit
kV imaging:
– kilovoltage digital radiography (kV DR)
– kilovoltage cone-beam CT (kV-CBCT)
– CT-on-rail
kV imaging:
– kV DR: ~ mGy (entrance dose)
– kV-CBCT
• Soft tissue:
2 - 8 cGy /acquisition
• Bone:
8 - 25 cGy /acquisition
Summary cont...
Imaged area is larger than the treatment field
Imaging-guidance procedures are more frequent than
diagnostic imaging
Repeated imaging procedures can sum up significant
dose to radiosensitive organs
kV DR imaging: high entrance dose
exit dose (~ 5% of entrance dose)
MV EPID imaging: exit dose (~50% of entrance dose)
Summary cont...
MV beam imaging:
– Dose resulting from MV-CBCT is comparable to that of multiple
portal imaging acquisitions
– Negligible difference between dose to bone and dose to soft
tissues
kV x-ray imaging:
– Dose resulting from kV-CBCT is much larger than that of multiple
kV DR acquisitions
– Dose to bone is 2-4 times higher than the dose to soft tissues
– Dose to bone marrow? (see Kawrakow et al WE-E-332-04)
What to do now?
Acknowledgements
Improve imaging technology (manufactures)
– Increase image quality and decrease the dose to patients
– Progress is being made
Matthew Deeley,
Use imaging guidance efficiently:
– Choose the procedure and the frequency that is most suitable for
the purpose
– Develop protocols for using image guidance procedures
– Pay attention to pediatric patients and imaged volume
Account imaging guidance dose for radiotherapy patients
– Calculate organ doses resulting from image guided procedures
– Account them as part of total dose to patients in radiotherapy
treatment planning systems
Vanderbilt University
Kenneth Lewis,
Vanderbilt University
Fitz-William Taylor,
visiting student from USMA at West Point
ACCRE
Vanderbilt Advanced Computing Center for
Research and Education